Journal of Volcanology and Geothermal Research 304 (2015) 253–264 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Subsurface structure of a maar–diatreme and associated tuff ring from a high-resolution geophysical survey, Rattlesnake Crater, Arizona Anita Marshall a,⁎, Charles Connor a, Sarah Kruse a, Rocco Malservisi a, Jacob Richardson a,LeahCourtlandb, Laura Connor a,JamesWilsona, Makan A. Karegar a a University of South Florida School of Geosciences, 4202 East Fowler Ave, Tampa, FL 33620, United States b Department of Earth-Space Physics, University of Indianapolis, 1400 East Hanna Ave, Indianapolis, IN 46227, United States article info abstract Article history: Geophysical survey techniques including gravity, magnetics, and ground penetrating radar were utilized to study Received 12 May 2015 the diatreme and tuff ring at Rattlesnake Crater, a maar in the San Francisco Volcanic Field of northern Arizona. Accepted 3 September 2015 Significant magnetic anomalies (+1600 nT) and a positive gravity anomaly (+1.4 mGal) are associated with Available online 9 September 2015 the maar. Joint modeling of magnetic and gravity data indicate that the diatreme that underlies Rattlesnake Crater has volume of 0.8–1km3, and extends to at least 800 m depth. The modeled diatreme comprises at least Keywords: two zones of variable density and magnetization, including a low density, highly magnetized unit near the center Phreatomagmatism fi Tuff ring of the diatreme, interpreted to be a pyroclastic unit emplaced at suf ciently high temperature and containing Magnetic sufficient juvenile fraction to acquire thermal remanent magnetization. Magnetic anomalies and ground Gravity penetrating radar (GPR) imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry GPR high magnetization, likely produced by energetic emplacement of hot pyroclastic density currents. GPR profiles San Francisco Volcanic Field on the tuff ring reveal long (~100 m) wavelength undulations in bedding planes. Elsewhere, comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse. Interpretation of these geophysical data suggests that Rattlesnake Crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed. The positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor. These dense magnetized bodies have excess mass of 2–4×1010 kg, and occupy approximately 5% of the diatreme by volume. Magnetic anomalies on the crater floor are elongate NW–SE, suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation. GPR imaging of the tuff ring also suggests that substantial land-slip may have occurred on the western rim, perhaps causing part of the tuff ring to collapse into the crater. Strong radar reflections indicative of well-developed weathering horizons are present as well. The techniques employed at Rattlesnake Crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option. © 2015 Elsevier B.V. All rights reserved. Introduction maars, which greatly augment what we can observe on the surface and contribute to our understanding of the structure and eruptive Explosive phreatomagmatic volcanism creates risk for the millions mechanisms of phreatomagmatic vents (Schulz et al., 2005; Mrlina of people that live within active volcanic fields around the world et al., 2009; Blaikie et al., 2014). Here we present new geophysical sur- (Chester et al., 2000). These eruptions occur when rising magma and veys and forward models of the sub-surface structure for Rattlesnake groundwater interact, and can produce craters 1–2 km in diameter Crater, one of many phreatomagmatic eruption sites in the San and more than 300 m deep (Wohletz and Sheridan, 1983; Lorenz, Francisco Volcanic Field (SFVF) of northern Arizona, USA. 2003; Lorenz and Kurszlaukis, 2007; Valentine and White, 2012). The Maar craters are underlain by diatremes that contain pulverized excavation of such craters, or maars, may result from one explosion or country rock and juvenile material produced during the eruption many, and the length of time between eruptions is highly variable (White and Ross, 2011; Valentine and White, 2012). The top of the (White and Ross, 2011). Geophysical survey methods provide valuable diatreme is generally assumed to roughly coincide with the diameter data about diatremes and related sub-surface features associated with of the surface crater, although the shape of the crater can change signif- icantly due to syn-eruptive and post-eruptive processes such as faulting, fi ⁎ Corresponding author. mass wasting, subsidence, and in- lling by sediments (White and Ross, E-mail address: [email protected] (A. Marshall). 2011). Previous geophysical surveys reveal that some maar–diatremes http://dx.doi.org/10.1016/j.jvolgeores.2015.09.006 0377-0273/© 2015 Elsevier B.V. All rights reserved. 254 A. Marshall et al. / Journal of Volcanology and Geothermal Research 304 (2015) 253–264 have complex structures that are not evident on the surface, including bimodal, but most volcanic vents erupt basalts (Priest et al., 2001). multiple eruption points, igneous dikes, buried lava lakes, faults and There are at least 12 vents that show evidence of phreatomagmatic subsidence features (Schulz et al., 2005; Mrlina et al., 2009; Blaikie activity within the SFVF; many of these sites have complex or mixed et al., 2012; Bolós et al., 2012). eruptive histories. Surrounding the crater, a tuff ring made of ejected material forms a Rattlesnake Crater is a basaltic maar and tuff ring located in the rim around the eruption site. Examination of the tuff ring can yield valu- southeast region of the SFVF (Fig. 1). The crater is elongate, approxi- able information regarding magma volatile content and composition, mately 1.4 km in diameter on the long axis, in a NW–SE direction. The and the duration of an eruption (Chough S.K., 1990; Sohn and Park, tuff ring surrounding the crater varies in height from approximately 2005; Brand and Clarke, 2009). Observations of erosional surfaces 60 m on the NE side to only a few meters high on the SW side, and is ob- within tuff rings are used to infer the number of eruptive phases a scured on the SE side by an overlapping scoria cone, called Rattlesnake vent may have produced, and the duration of repose between them Hill. The presence of tephra from Sunset Crater (Ort et al., 2008), and (McPhie et al., 1990; Zimmer et al., 2010). Analysis of exposed deposits the Brunes-age magnetic orientation associated with Rattlesnake Hill can also be used to estimate eruptive energy (Vazquez and Ort, 2006; lavas (Tanaka et al., 1986) place the age of Rattlesnake Crater between van Otterloo and Cas, 2013). However, many stratigraphic studies of 900 and 780,000 yBP. phreatomagmatic sites are limited by the availability of exposed units Rattlesnake Crater is constructed on top of substantial lava flows, in outcrop. tens of meters thick that crop out in the area surrounding the tuff ring. Utilizing gravity and magnetic surveys is a well-established Based on coring in the area, the sub-surface stratigraphic column com- approach to studying phreatomagmatic eruption sites (Cassidy et al., prises Paleozoic sedimentary units identified in descending order as 2007; López Loera et al., 2008; Mrlina et al., 2009; Bolós et al., 2012; the Kaibab, Toroweap, Coconino, Schnebly Hill, and Supai Formations Skácelová et al., 2012). Similarly, GPR has been widely utilized in studies (Hoffmann et al., 2006). The uppermost stratigraphic unit, the Kaibab of volcanic tephra and surge deposits (Russell and Stasiuk, 1997; Formation, is a fractured (Gettings and Bultman, 2005) and karsted Gómez-Ortiz et al., 2006; Gomez et al., 2008; Kruse et al., 2010; (Montgomery and Harshbarger, 1992)Permianlimestonewitha Courtland et al., 2012, 2013), and of phreatomagmatic deposits maximum thickness of 200 m in the area. Underlying the Kaibab, the (Cagnoli and Ulrych, 2001). In this paper we use all three techniques Toroweap Formation, a mixed variety of near-shore clastic units, is to constrain the geometry, volume, and facies of the diatreme beneath less than 100 m thick. The Coconino Sandstone, a sequence of cross- Rattlesnake Crater and its tuff ring. bedded fine-grained sandstone 400–500 m thick, is the primary water-bearing unit. The thin (tens of meters) Schnebly Hill Formation Rattlesnake Crater and the San Francisco Volcanic Field both interfingers with and underlies the Coconino and is made of very fine grained mudstones, limestone and dolomite units. The well logs The SFVF of northern Arizona is an active volcanic region containing of Hoffman et al (2006) end in the Supai Formation, sometimes grouped more than 600 volcanic vents within 4700 km2 (Priest et al., 2001). The with a number of lower units as “Redbeds”, which are a collection of dis- SFVF is a Colorado Plateau field (Condit et al., 1989), and the locus of ac- tinctively colored alternating units of sandstone, siltstone and mudstone. tivity within the field has shifted from west to east with time, reflecting The entire tuff ring is covered by weathering products and fall the motion of the North American Plate (Tanaka et al., 1986). The SFVF is deposits, with the important exception of a band of outcropping units Fig. 1. Data acquisition locations for the geophysical study of the Rattlesnake Crater complex (blue star, inset), one of hundreds of volcanic vents (red dots, inset) within the SFVF, Arizona. The complex includes Rattlesnake Crater, a maar surrounded by a tuff ring and bounded to the SE by Rattlesnake Hill, a scoria cone. White lines show the track of the ground magnetic survey (Fig. 3); black dots indicate gravity survey points (Fig.